1. Field of the Invention
The present invention relates to a liquid droplet jetting apparatus which jets liquid droplets.
2. Description of the Related Art
A liquid droplet jetting apparatus, which is structured such that a control circuit board is provided at a position away from a liquid droplet jetting head which jets liquid droplets, and a signal for jetting the liquid droplets is transmitted from the control circuit board to the liquid droplet jetting head via a wire formed on a flexible wiring board, has hitherto been known.
In the liquid droplet jetting apparatus having a structure as described above, when the signal is transmitted from the control circuit board to the liquid droplet jetting head via the wiring board, a noise is radiated from each wire of the wiring board, to a surrounding area. This radiated noise has an effect on peripheral equipment, and also becomes a cause of malfunction of the apparatus. Therefore, a liquid droplet jetting apparatus having the following structure has been proposed in order to reduce such radiated noise.
For instance, a liquid droplet jetting apparatus disclosed in Japanese Patent Application Laid-open No. 2007-196448 includes a head which jets liquid droplets, a head control section which controls the head, and a drive signal generating section which generates a drive pulse to be applied to the head. Pixel data of two bits (four gradations namely, without dot, small dot, medium dot, and large dot) which is related to dots to be formed on a paper is inputted to the head control section from an ASIC upon being synchronized with a clock signal, via the wiring board. Moreover, based on the inputted pixel data, a desired pixel (dot) is formed by applying a predetermined interval portion corresponding to the pixel data, among drive pulses generated in the drive pulse generating section, to a piezoelectric element (liquid droplet jetting portion). The drive pulse generated in the drive pulse generating section is also outputted to the head control section via the wiring board. When there is no jetting of the liquid droplets from the head, the drive signal generating section outputs a constant voltage instead of the drive pulse to the head control section to reduce the noise radiated from the wiring board.
However, even at the time of transmitting the pixel data, which is formed of bit data of a predetermined number of digits related to a dot to be formed on the paper, from the ASIC to the head control section via the wiring board, the noise is radiated to the surrounding from the wires of the wiring board. More concretely, when the bit data forming the pixel data is transferred serially and continuously from the ASIC to the head control section, if there occurs a switching of “0” and “1” between (in) the bit data, due to a change in a signal level, the current flows instantaneously. At this time, the noise is radiated from the wires.
Moreover, when gradation is increased for an image formation of higher quality, the number of bits forming one pixel data increases (for example, becomes 3 bits or more), and the total number of bit data to be transmitted by the wiring board for one jetting timing increases. For transferring the increased bit data in a short time by increasing a data transfer speed, it is necessary to increase a clock frequency. However, with the increase in the clock frequency, the noise radiated from the wires at the time of data transfer tends to increase.
The abovementioned content will be described below more concretely. It is possible to express an electric field strength Ec (in other words, strength of the radiation noise) at a predetermined position around the wire at a distance d from the wire by the following expression (1).Ec=0.628×10−6×i[A]×f[Hz]×L[m]×(K×1)/d[m]  (1)
where, i is a magnitude of the electric current, and is proportional to the number of switching of a bit value (“0” and “1”). Moreover, f is a frequency of a transfer clock, L is a length of the wire, and K is a constant. From the expression (1), it is evident that as the switching frequency (the number of switching) (electric current i) of the bit value and the clock frequency f increases, the magnitude (strength) of the electric field increases. In this manner, when the clock frequency is increased (made higher) from increasing the transfer speed, an effect of the radiation noise generated from the wires due to the transfer of the bit data cannot be neglected. It is desirable to reduce the radiation noise generated by such factor.